1. Field of the Invention
This invention relates to an apparatus and process for sleeving damaged heat exchanger tubes in nuclear steam generators, and is specifically concerned with a device and method for expeditiously positioning a sleeving device and feeding a sleeve within a heat exchanger tube so as to enable a greater number of tubes to be sleeved in a given period of time.
2. Description of the Prior Art
During the life of a nuclear steam generator, some of the heat exchanger tubes become corroded, damaged, or cracked. These tubes must be repaired if the entire service life of the steam generator is to be realized. These repairs are accomplished by installing a tubular sleeve within the tube which traverses the damaged portion of the tube which traverses the damaged portion of the tube walls. The sleeve is dimensioned so that there is little clearance between the outer walls of the sleeve and the inner walls of the tube, and the ends of the sleeve are expanded or brazed to the inner walls of the tube to form a hydraulic "bridge" across the damaged portion of the tube. Such sleeving is accomplished by a succession of tools which are suspended from the tube sheet within the steam generator. These tools generate a number of reactive forces in performing their functions and consequently, they must be firmly and reliably secured to the tube sheet. The purpose of the instant invention is to expeditiously secure a sleeving tool to the underside of a tubesheet.
Devices for inserting and positioning reinforcing sleeves within the heat exchanger tubes of steam generators are known in the prior art. One such device developed by the personnel of the Westinghouse Electric Corporation is capable of feeding such reinforcing sleeves through the open ends of the tubesheet of a nuclear steam generator by means of a pair of hydraulic grippers, each of which includes an expandable bladder. The frame of this device is light enough to allow it to be manipulated within the radioactive, primary side of a nuclear steam generator by a remotely operable service arm (or ROSA) that has also been invented and developed by personnel of the Westinghouse Electric Corporation. Once situated, the sleeve-feeding mechanism of this device is locked into position adjacent to the sleeve to be repaired by one or more hydraulically operated cam-lock devices that are insertable within and expandable against the open ends of tubes that are not being repaired. After the device has been secured into a proper position, the hydraulic grippers are actuated. The feeding of the sleeve is accomplished by alternatively actuating and deactuating the hydraulic grippers, and by reciprocating one of the grippers relative to the other. A complete description of this sleeve loading mechanism is set forth in U.S. Pat. application Ser. No. 785,291 filed Oct. 3, 1985 now U.S. Pat. No. 4,771,526 , by Thomas E. Arzenti and William E. Pirl, and a complete description of a hydraulically operated cam-lock device for mounting such tools within such steam generators is described in U.S. Pat. No. 4,728,217 issued Mar. 1, 1988 to David J. Fink. Both of these applications are assigned to the Westinghouse Electric Corporation, and both are expressly incorporated herein by reference.
Generally speaking, the ultimate purpose of such maintenance devices is to prevent radioactive water from the primary side of the generator from leaking through the cracked walls of corroded heat exchange tubes. If such a leak should occur, radioactive water from the primary side would contaminate the nonradioactive water present in the secondary side of the generator. Since the water in the secondary side is used to form the steam which ultimately drives the electric turbines in the plant, it is important that this water remain uncontaminated.
Such sleeving operations have proven to be very effective in extending the useful lifetime of the nuclear steam generator. Unfortunately, they are also quite expensive since the steam generator has to be completely shut down and taken off-line. Such downtime can cost the utility involved over $1,000,000 per day in lost revenues. It is therefore desirable that such sleeving operations be accomplished as rapidly as possible. While the sleeve loading tool described and claimed in the previously mentioned U.S. Pat. application Ser. No. 785,291 is among the fastest and most effective tools known for positioning reinforcing sleeves across corroded sections of heat exchanger tubes, it has several design limitations that prevent it from positioning as many sleeves as possible within a given unit of time. For example, the entire tool must be robotically re-positioned in the tubesheet whenever a new tube is to be sleeved.
A device which provides a more rapid sleeving operation is described in U.S. Pat. application Ser. No. 006,845 filed Jan. 27, 1987 now U.S. Pat. No. 4,829,648, by Thomas Arzenti et al. and assigned to the Westinghouse Electric Corporation, and which is completely incorporated herein by reference. Here, a double action hydraulic cylinder that is capable of translating the frame which carries a pair of hydraulically operated grippers, one of which is reciprocably movable with respect to the other, into engagement with the tube sheet after the assembly has been pivoted into alignment with the damaged tube. The device is capable of servicing a number of tubes with a single insertion of the mounting mechanism.
However, during the insertion of the sleeve, undesirable reverse forces are exerted on the tube being serviced, the adjacent tubes as well as, the frame and the robotic arm which positions the sleeving assembly beneath the sheet. These forces result from the close fit between the sleeve and the tube itself. During the feeding of the sleeve, the smallest misalignment between the sleeve and the tube may cause the sleeve to jam within this tube. To relieve such a jam, the maintenance operator may have to apply forces which create even greater stresses on the tubes and the tool of Arzenti. This greater force exerted by the tool is absorbed by the positioning arm as well as the tube being serviced and the adjacent tubes by way of the gripping units, and may result in further damage to these tubes as well as damage to the positioning arm. Also, because there is often not a smooth feeding of the sleeve into the tube being serviced, the time required to complete the servicing of the tube is increased.
Clearly, there is a need for an improved sleeving device capable of installing reinforcing sleeves in heat exchanger tubes in shorter time periods while minimizing the loads exerted on the positioning tool and the tubes of the tubesheet. Ideally, such a device should be easily and remotely manipulable within the radioactive environment of the primary side of the generator by means of commercially available robotic arms. Furthermore, it would be desirable if the operation of the device provided reliable positioning of the tool with respect to the tubesheet and provided for the smooth insertion of the sleeve so that the entire operation was substantially accelerated, and the maintenance operator's exposure to potentially harmful radiation was minimized.